It is well known that the desiccant in an air drier, preferably for use on a vehicle, is regenerated by dry air that is passed backwards through the desiccant and out in the atmosphere thereby removing the moisture gathered by the desiccant, which thereafter will be ready for drying more air coming from the compressor in the system. The regenerating occurs when the compressor has been shut off because no more air is needed in the system.
In a single-tower air drier system there are different principles to govern the regeneration. It is presently preferred to regenerate during a certain time, say 20-30 seconds, after each drying sequence. This controlled time for passing dry air from the air tank through a retarding valve (by-passing a check valve) to the desiccant cartridge may be obtained in different ways.
One method of governing the regeneration time is disclosed in DE-A-3 139 682, where the opening of the valve is electrically controlled by a timer or the like. Another method is disclosed in EP-A-36 569, where a completely pneumatic retarding is obtained with a relatively complex and sensitive system having a spring biased piston, chambers and orifices.